1. Field of the Invention
The present invention relates to a magnetic braking apparatus, and in particular, to an improvement of the magnetic braking apparatus to enable detection of a load applied thereto, and to a tension control system of a roll diameter proportion type employing the magnetic braking apparatus.
In a process of letting off or taking up paper or web, in order to prevent slack or excessive tension of the paper or the web, the tension is controlled to maintain constant tension. In such a process, a magnetic braking apparatus having a sensor capable of measuring a load or a braking torque is very beneficial.
2. Description of the Prior Art
A prior art electromagnetic powder brake as a magnetic braking apparatus which is incorporated in a tension control system is shown, for example, in FIG. 1.
With reference to FIG. 1, a yoke 1 extending circumferentially and an exciting coil 2 accommodated in an annular recess of the yoke 1 constitute field body or an electromagnet portion which produce a magnetic field. A rear bracket 3 and a front (input side) bracket 4 support the field body stationary by a strut 4a which is secured to a base 4b . A cylinder 7 having a side plate 5 is secured to an input shaft 6 which is supported by bearings 9 rotatably. An outer peripheral wall of the cylinder 7 is divided into two parts by an interrupting ring 8 of a non-magnetic material. A stationary rotor 10 is secured to the rear bracket 3. A magnetic powder 11 is sealed in an air gap between an outer peripheral surface of the rotor 10 and an inner peripheral surface of the cylinder 7.
In operation, when the coil 2 is excited by a DC current, magnetic flux is generated and flows along a path shown by the dotted line, from the yoke 1 to cylinder 7 to magnetic powder 11 to rotor 10 to magnetic powder 11 to cylinder 7 and to yoke 1. As a result, the magnetic powder 11 is magnetized by the magnetic flux and particles of the magnetic powder 11 chain together and solidify so that a braking force is applied from the stationary cylinder 7 to the input shaft 6 through the magnetic powder 11. This braking force corresponds to the magnitude of the current supplied to the exciting coil 2, and the braking action is performed with a slip produced between the cylinder 7 and the rotor 10.
However, the following problems are involved in the prior art magnetic braking apparatus.
Since the prior art magnetic braking apparatus is not provided with a device for detecting a braking torque or detecting a load applied to the magnetic braking apparatus, when it is desired to measure the braking torque or the load, it is necessary to use separate torque measurement equipment or a load detecting device.
In particular, recently, since the magnetic braking apparatus such as an electromagnetic powder brake is incorporated in an automatic control system requiring a high accuracy including a tension control system, it is indispensable to detect a torque or a load in order to perform a feedback control.
In the prior art magnetic braking apparatus, in order to meet the aforementioned requirements, a load detecting device or the like must be equipped separately, and the associated facility will become large and expensive
In order to improve the above-mentioned drawbacks, a proposal by the applicant's company is disclosed in Japanese Patent Publication No. 57-56687. In this prior art motive power measuring apparatus employing an electromagnetic powder brake, as shown in FIGS. 2 and 3, an electromagnetic powder brake 51 is driven by a prime motor (not shown) to be measured such as an electric motor (not shown), and it absorbs generated motive power generated by the prime motor. The electromagnetic powder brake 51 includes a rotary shaft 52, a rotor 53 secured to the rotary shaft 52 and having a substantially T-shaped longitudinal cross section in its half part, and non-magnetic rings 54, 54 forming two halves of a cylindrical portion of the rotor 53. A stator 55 (which is also rotatable as described later) includes outer yokes 56, 56 inner yokes 56', 56', exciting coils 57, 57, connecting rings 58, 58 for connecting the outer and inner yokes 56 and 56' respectively, and a coupling ring 59. The stator 55 is rotatably supported by supporting members 60, 60 secured to the stator 55, bearing housings 61, 61, support table 62, and bearings 63, 63. Bearings 64, 64 support the rotor 53 and rotary shaft 52 rotatably with respect to the stator 55. Magnetic powders 65, 65 are sealed on an outer and inner surfaces of the cylinder portion of the rotor 53.
A torque arm 70 made of metal is secured to an outer surface of the stator 55 and is extending radially from a center portion in an axial direction. A load transducer 66 is fixed between the torque arm 70 and a fixing member 68 which is secured to the support base 62. The load transducer 66 is expanded or compressed within a certain range under a tension or compression, and transforms a reaction force into a load. As a result, the rotation of the brake 51 is limited in a range in which the load transducer 66 is allowed to expand or to be compressed. A tachometer generator 69 is coupled to the rotary shaft 52.
In the case where the rotor 53 is rotating, when the coils 57, 57 are excited, closed circuits of magnetic flux are formed, and a coupling force is produced between the stator 55 and the rotor 53 through chained and solidified magnetic powders 65, 65. As a result, the stator 55 tends to rotate, however, since the rotation of the stator 55 is limited by the load transducer 66 connected to the stator 55 through the torque arm 70, a braking force is applied to the rotor 53, and thus, to the prime motor to be measured. A reaction force of this braking force is applied to the load detector 66 through the torque arm 70. Accordingly, when an output signal of the load transducer 66 is amplified and displayed on a load display device (not shown), the braking force can be measured as rotation moment which corresponds to the product of the braking force and a length of the torque arm 70. Thus, the motive power of the prime motor to be measured can be measured from a relation between a rotational speed to the rotary shaft 52 obtained by the rotation tachometer 69 and the rotation moment.
However, in this prior art motive power measuring apparatus, the following problems are involved. Specifically, in this apparatus, since it is the purpose to obtain the generated motive power and not the load itself, the accuracy of the load measurement is not so high. Furthermore, in the structure of detecting the load, the braking force which is transmitted as a reaction force from the stator of the electromagnetic powder brake is once transmitted to the torque arm made of metal. Then, the braking force is transmitted to the load transducer 66 which is installed horizontally on the support base 62 through the fixing member 68. As a result, the number of parts is increased, and the accuracy is low due to indirect detection of the load. This accuracy is not satisfactory in the field of a tension control system for letting off a web.
A prior art tension control system of the roll diameter proportion type which incorporates a magnetic braking apparatus of the types as described above is shown in FIG. 4. The tension control system (e.g., a tension controller PCA-101 of Shinko Denke Kabushiki Kaisha) is used in a let-off (unwinding) process of a web.
With reference to FIG. 4, the tension control system is used to let off, unwind, or feed a material, for example, a web, or paper 100 while adjusting the tension of the web. The web 100 is let off from a driving (let-off) reel 101. A rotation detector 102 including a detection disk 103 and a proximity sensor 104 is provided. The detection disk 103 is fixed to the driving reel 101 coaxially and has a projection 103a on a periphery of the detection disk 103. The proximity sensor 104 detects passing of the projection 103a and generates a pulse for each rotation of the detection disk 103.
A driven roller 105, and pinch rollers 106 and 107 are provided along the path of the web 100. A take-up reel 111 is also provided to take up the web 100. A pulse generator 108 is coupled to the pinch roller 107, and it generates pulses whose number corresponds to a take-up speed of the web 100. A controller 109 calculates a diameter (roll or thickness) of the wound web 100 based on a time period of one rotation of the disk 103 and the number of pulses during one rotation supplied from the pulse generator 108, and based on the following relationships among the torque, tension, and the diameter, supplies an exciting voltage Vf required to obtain a predetermined tension to an exciting coil 110a of a magnetic braking apparatus 110 such as an electromagnetic powder brake or the like.
FIG. 5 is a block diagram illustrating the principles of operation of the prior art tension control system including the controller 109 and the magnetic braking apparatus 110, and F is the tension, D is the let-off diameter, and T is the torque. In this case, supposing that braking torques at the start and finish of the let-out operation are represented respectively by Ts and Te, The braking torques are expressed as follows. EQU Ts=(Fmax.times.Dmax/2).times.10.sup.-3 (kgm) (1) EQU Te=(Fmax.times.Dmax/2).times.10.sup.-3 (kgm) (2)
Where, Fmax is a maximum set tension, Dmax and D min are respectively a maximum let-off diameter and a minimum let-off diameter.
Furthermore, in the prior art tension control system as shown in FIG. 4 and employing the aforementioned magnetic braking apparatus the following problems are involved.
With reference to FIG. 6, when the tension is maintained at a set tension, the torque-roll diameter characteristic is represented by a curve (A) in which the torque T is proportional to the roll diameter (diameter of the web wound about the let-off reel) D. However, the braking torque which is actually applied to the driving reel 101 by the magnetic braking apparatus 110 is changed as shown by the curve (B). Namely, in a region wherein the roll diameter D is small, the braking torque T is smaller than the set value, and in a region wherein the roll diameter D is large, the braking torque T is larger than the set value. As a result, the control for maintaining the tension at a constant value cannot be performed properly. This drawback becomes especially significant when the electromagnetic powder brake is used as the magnetic braking apparatus, and the utilization of the electromagnetic powder brake is rather disturbed due to the required accuracy irrespective of the fact that the electromagnetic powder brake is excellent in the slip characteristic.
Furthermore, in the prior art tension control system, a part of the braking torque is dissipated as a mechanical loss and the like, and the tension does not become constant.